Interaction thresholds in Er:YAG laser ablation of organic tissue

Because of their unique properties with regard to the absorption in organic tissue, pulsed Er:YAG lasers are ofinterest for various applications in medicine, such as dentistry, dermatology, and cosmetic surgery. The relativelylow thermal side effects, and surgical precision of erbium medical lasers have been attributed to the micro-explosive nature of their interaction with organic tissue. In this paper, we report on preliminary results of ourstudy of the thresholds for tissue ablation, using an opto-acoustic technique. Two laser energy thresholds for theinteraction are observed. The lower energy threshold is attributed to surface water vaporization, and the higherenergy threshold to explosive ablation of thin tissue layers.k INTRODUCTIONBecause it ablates and cuts tissue with surgical precision and with minimal secondary thermal damage, theEr:YAG laser is of special interest in medicine. The erbium laser wavelength of 2.94 pm is strongly absorbed bywater which is the major constituent of organic tissue. The corresponding small penetration depth, combinedwith the high power of the short Er:YAG pulses gives the laser unique capability for precise tissue ablation.Because of its fundamental advantages, many experts believe that the erbium laser will become an importanttool for various treatments on hard, and soft human tissues.Extensive pre-clinical research, and clinical investigations of the use of the erbium laser for the ablation ofcartilage, bone,{1] dental tissue,[2] and skin,[3] are being carried out. Thus, for example, as part of the on-goingresearch involving experts from Department of Oculopastic Surgery of University Eye Clinic, and UniversityDermatology Clinic, in Ljubljana, the Er:YAG laser (Fotona Twinlight, Model 210 A) has been applied fortreating various benign epitlielial skin lesions, as well as for skin resurfacing and aesthetic surgery.[4-5} Thelesions were irradiated with single pulse energies 50-500 mJ, repetition rates 2-15 Hz, and spot diameters 1.5-8mm. In the study, 42 patients have been treated so far for benign skin lesions (seborrhoic warts, plane warts,